Digital projectors often include micro-displays that include arrays of pixels (e.g., 1280×1024, etc.) Each pixel usually includes a micro-electromechanical system (MEMS) device, such as a micro-mirror, liquid crystal on silicon (LcoS) device, interference-based modulator, etc. A micro-display is used with a light source and projection lens of the digital projector. The micro-display receives light from the light source. When the pixels of the micro-display are ON, the pixels direct the light to the projection lens. When the pixels are OFF, they do not direct the light toward the light source, e.g., they may direct the light from the light source away from the projection lens, absorb the light, etc. The projection lens images and magnifies the micro-display.
Many micro-displays include a transparent, e.g., glass, cover for packaging, e.g., sealing and/or protecting, the pixels. However, when the pixels are OFF, the transparent cover can reflect some light to the projection lens. This degrades the “Black/White Contrast ratio” that is often defined as the ratio of the light imaged by the projection lens when all of the pixels in the micro-display are ON to the light imaged by the projection lens when all of the pixels are OFF and is a measure of the blackness of the projector's black state.
The pixels of many micro-displays are formed on a semiconductor substrate using semiconductor processing methods, and the transparent cover is adhered to a layer, e.g., an oxide layer, formed on the substrate overlying the pixels. However, the interface between the layer and the cover may have a substantially different index of refraction than either the layer or the cover, e.g., due to air gaps etc. This causes some of the light that reaches the cover to be reflected by the cover to the projection lens, which acts to reduce the contrast ratio.